Multiple reflection of energy to effect increased gain



Sept. 3, 1968 H. w. EHRENSPECK 26,443

REFLECTION ANTENNA EMPLOYING MULTIPLE DIRECTOR ELEMENTS AND MULTIPLEREFLECTION OF ENERGY TO EFFECT INCREASED GAIN Original Filed May 11,1959 TOR. M/WMAMIAI m INVEN United States Patent Office Re. 26,448Reissued Sept. 3, 1968 26,448 REFLECTION ANTENNA EMPLOYING MULTIPLEDIRECTOR ELEMENTS AND MULTIPLE RE- EIAEEITION OF ENERGY T EFFECTINCREASED Hermann W. Ehrenspeck, 94 Farnham St., Belmont, Mass. 02178Original No. 3,122,745, dated Feb. 25, 1964, Ser. No. 812,565, May 11,1959. Application for reissue Feb. 8, 1965, Ser. No. 448,903

4 Claims. (Cl. 343-819) (Granted under 'lh'tle 35, US. Code (1952), sec.266) Matter enclosed in heavy brackets [1 appears in the original patentbut forms no part of this reissue specification; matter printed initalics indicates the additions made by reissue.

The invention described herein may be manufactured and used by or forthe United States Government for governmental purposes without paymentto me of any royalty thereon.

This invention relates generally to directional antennas and moreparticularly to a modification of slow wave antennas to produce areflection of energy from an array to cause it to traverse the array atleast once before it is radiated and thereby increase gain.

The gain of slow wave antennas depends on the phase velocity of thesurface wave travelling along it and the length of the antenna; however,for a given length there is an optimum phase velocity beyond which thegain decreases, therefore, for adjustment of antennas at optimum phasevelocity, the gain becomes proportional to the antenna length.

The utilization of the concept of this invention whereby the use of areflection arrangement to cause a traverse of at least part of theenergy of an endfire slow wave array back along the array has been foundto increase the effective length of the array and, therefore, cause anincrease in antenna gain. The gain increase thus achieved isaccomplished without extensive modification of the antenna or physicallyincreasing the length.

Accordingly it is an object of this invention to provide a novel methodand means for increasing gain of all types of slow wave antennas.

It is another object of this invention to provide an antenna of one-halfor less of the usual antenna length for a given gain.

It is still another object of this invention to provide at least doublegain for an antenna of a given length over that previously attainable.

It is a further object of this invention to provide a novel antenna witha very high ratio of front to back radiation.

It is a still further object of this invention to provide a structure toincrease antenna gain while maintaining a feed at only one point.

Another object of this invention involves the provision of a novelendfire array arrangement that allows sharp narrow beam-width patternswith reduced sidelobes.

Still another object of this invention involves the provision of anantenna utilizable at high or low frequencies.

A further object of this invention involves the provision of an antennasuitable for flush mounting.

A still further object of this invention involves the construction of anovel endfire antenna of currently available material that [lendthemselves] lends itself to standard mass production manufacturingtechniques and is of less cost than antennas of similar gain.

These and other advantages, features and objects of the invention willbecome more apparent from the following description taken in connectionwith the illustrative embodiments in the accompanying drawings, wherein:

FIGURE 1 is a schematic representation of a conventional endfire antennawith the main lobe of its pattern;

FIGURE 2 is a schematic representation of the backfire antennaembodiment of my invention with the main lobe of its pattern;

FIGURE 3 is a schematic representation of the embodiment of FIGURE 2adapted for flush mounting; and

FIGURE 4 is a schematic representation of a multiple reflection endfireantenna embodiment with the main lobe of its pattern.

Referring to FIGURE 1, a conventional endfire Yagi antenna is shownwherein the feed F at one end excites the array and the energy travelsalong the array by means of directors D with a phase velocity slowerthan that of light. Apart from a negligible amount radiated directlyfrom the feeder F, the energy is radiated from a virtual aperture (shownin dashed lines) located at the termination of the array. The concept ofthe virtual aperture is more fully explained in my copending applicationSerial No. 719,698, filed Mar. 6, 1958, now Patent No. 3,096,- 520titled Endfire Array, wherein the virtual aperture at the end of anarray includes the field at which power levels are from maximum to 20 dbbelow maximum. In most endfire arrays a linear reflector R is mountedbehind the feeder, as shown in FIGURE 1, to increase the gain in theforward direction. The energy travels along the array in the directionof the arrows and radiates with the pattern (sidelobes omitted)indicated.

The phase front of a traveling wave lies substantially in a plane at theradiating end of an endfire array when the antenna is adjusted foroptimum gain in the forward direction. The concept of this invention,embodied in the representation labeled FIGURE 2, allows the travelingwave to impinge on and be reflected by a planar reflector M such thatthe wave now travels along the array for a second time in the directiontoward the feed. Then, the array of FIGURE 1 as modified by the use ofthis invention as shown in FIGURE 2 utilizes a feed F, a reflector R,and a slow wave director structure comprising directors D, and has avirtual aperture V. By placing the planar reflector M at what wouldnormally be the end of the array, the new virtual aperture now appearsat the feed end of the array. Reflectors R and M are spaced at theirusual positions with respect to the array at approximately )./4. Themajor part of the energy first travels along the array as shown by theinner arrows and travels in the opposite direction as indicated by theouter arrows to be radiated with a higher gain, narrower beamwidthpattern, as shown, in a diverse direction to that of FIGURE 1. Themirror action of the planar reflector M differs from the usual use andaction of plane reflectors with linear or slow wave antennas. With alinear antenna a planar reflector causes constructive interferencebetween the original antenna wave and the reflected one to increase theforward gain. Planar reflectors with slow wave structures have been usedin the past to prevent backlobes. However, the waves impinging on thereflector in this case are not slow waves since the usual position isbehind the feed. The mirror action of reflector M, since there is nodirect wave with which to interefere as in the linear antenna case, andsince it is situated in the main wave channel of a slow wave structureto reflect the slow wave energy, causes all the energy in the channel toretravel along the array thus creating a large increase in gain. Thisincrease in gain is explainable by the fact that the retravel makes theantenna act like one double its length. In FIGURE 2 the gain would be atleast equal to a factor of two except that reflector R may tend toreduce the gain factor slightly] as is the increase in eflective lengthof the antenna. The narrow pattern with a high ratio of front to backradiation and decreased sidelobes with the attendant increase in virtualaperture V requires an adjustment of directors D for a new lengthsuitable for an array which would normally be double in length. Linearreflector R, it has been found, causes a reflection of the energy towardreflector M; however, its effect on the slow wave returning down thearray is to cause only negligible perturbations in the field. Element Mis generally flat and should be as large as the virtual aperture formaximum gain; however, a slightly curved plate corresponding to theplane of the phase front in the virtual aperture or mesh screen orclosely spaced linear elements may be used to form this reflector.

FIGURE 3 is an example of the flush mounting of the antenna of FIGURE 2.In this figure, as in all the figures, like elements have the samedesignations. The feed F in this application is enclosed in a hollowtube T which supports the array and its elements and feeds the feederelements which are insulated from tube T.

The backfire antennas of FIGURES 2 and 3 radiate in a direction reversefrom normal with an effective length of double and a gain increase of atleast 3 db above a conventional endfire array. A modification of thebackfire antenna allows for further gain increases by utilizing multiplereflection.

The multiple reflection concept is embodied in the schematicrepresentation of FIGURE 4 where a feed F feeds a slow wave directorstructure comprising an array of directors D in front of which is placeda partial reflector R, while a reflector M is placed behind the feed, asshown. This structure modifies the principle of the backfire antenha inthat only a part of the energy travelling along the antenna from thefeed end to the output end is reflected by reflector R while theremainder is radiated in the normal endfire direction. As in the conceptdescribed relative to the backfire antenna, the reflected portion of theenergy travels a second time along the antenna but in the oppositedirection until it impinges upon its feed reflector M. In the multiplereflection antenna, M is a planar reflector which must be of such sizeas to reflect back as much of the surface wave energy possible and formaximum gain is of the same size as the virtual aperture. The energythen travels a third time along the antenna; this time in the normalendfire direction. After reaching the antenna end it is again partlyradiated at the virtual aperture V and partly reflected back, as before.This process continues until all the energy has been radiated.

Maximum gain of the multiple reflection antenna with a mirror reflectorM of the same size as the virtual aperture requires optimization of twoparameters; the reflectivity of the partial reflector R at the radiatingend, and the phase velocity along the antenna which is accomplished byadjustment of the length of directors D in accordance with the neweffective length of the antenna.

Thus reversal of elements M and R of the backfire antenna and making Rproduce perturbations in the slow wave produces increased gain over thebackfire antenna. The multiple reflection antenna has greater structuralsimplicity and strength since the feed is located at the same end as thereflector plane.

The partial reflector R or the planar reflector M may be of solid metal,metalized plastic, screening material, or of closely spaced rods eitherparallel or radially mounted. For optimized gain the individual elementsD are decreased from the optimum value for a conventional endfireantenna for phase adjustment.

The multiple reflection principle may be applied to the 4 backfireantenna to increase its gain by utilizing a partial reflector ratherthan a linear reflector in order to cause a partial reflection backalong the array in accordance with the action described relative toFIGURE 4.

Although the invention has been described relative to particularembodiments, it will be understood to those skilled in the art that theinvention is capable of a variety of alternative embodiments, forexample, the methods of increasing gain are applicable to all types ofslow wave antennas such as the dielectric rod, cigar antenna, etc.Furthermore, the teachings of my aforementioned copending applicationare applicable to the backfire and multiple reflection antennas.

I intend to be limited only by the spirit and scope of the appended[claim] claims.

What is claimed is:

1. Means for increasing the gain and decreasing the sidelobes of amultiple director type of antenna array comprising a planar reflectorpositioned at what would normally be the emitting end of the series ofdirector elements, for reflecting energy back along said directorelements in reverse sequence, for emission at the end of said arraywhere the energy traverse began.

2. A backfire antenna comprising a slow wave director structure, a feedlocated with respect to said structure to transmit energy therealong,reflecting means for reflecting all of the energy at one end of saidstructure back along and bound to said structure with a phase shift atsaid reflecting means, and reflector means at the other end of saidstructure for reflecting part of the energy transmitted directly fromsaid feed back along said structure and part 0 the energy reflected fromsaid reflecting means, the energy at said reflector means being in aninphase relationship for maximum gain, the part of the energy notreflected by said reflector means being radiated into free space.

3. A backfire antenna as defined in claim 2 which produces an increasedvirtual aperture,- the size of said reflecting means being as large asthe virtual aperture.

4. A multiple reflection antenna comprising a slow wave directorstructure, a feed at one end of said director structure, a planarreflector at the end 0 said slow wave director structure behind saidfeed for reflecting the energy in the wave channel of said antenna backalong said structure, and partial reflector means at the end of saidstructure opposite said feed end for reflecting part of the reflectedslow wave energy back toward said first-mentioned reflector andradiating part of the slow wave energy.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS 2,822,536 2/1958 Sandretto 343840 2,841,792 7/1958Edson 343819 2,407,057 9/1946 Carter 343840 2,627,028 1/1953 Nowak343761 FOREIGN PATENTS 741,897 12/1955 Great Britain. 751,249 6/1956Great Britain.

ELI LIEBERlMAN, Primary Examiner.

